doc: gsg: Add second note about different Linux distributions

The initial note about the install guide for different distributions
can be easily overlooked. Add a second note to make it more prominent
in the first step.

Signed-off-by: Timon Skerutsch <zephyr@diodes-delight.com>

Author: Timon Skerutsch

doc/develop/getting_started/index.rst

@@ -23,6 +23,7 @@ Click the operating system you are using.
    .. group-tab:: Ubuntu
 
       This guide covers Ubuntu version 20.04 LTS and later.
+      If you are using a different Linux distribution see :ref:`installation_linux`.
 
       .. code-block:: bash